This invention concerns a tie rod for a rock securing system.
When building cavities into rock or when removing rock walls, forces are generated which tend to move the rock towards the free space. To prevent this rock anchoring units are installed at the ends of the boreholes and are tightened at the beginnings of the boreholes or at the free rock wall by an anchor plate and a draw bolt.
A problem arises with the anchoring at the end of the borehole, and several suggestions have already become known in this context. According to German Pat. No. 1,117,071, a rigid crescent-shaped wedge is placed against the tension element or the anchor bolt which is held in the axial direction by its shape and friction. The outer surface of the wedge is inclined towards the axis of the tension element and interacts with the inner jacket surface of a loose wedge, also having a crescent-shaped cross-section, such that with a tension force the rigid wedge is shifted vis-a-vis the loose wedge and thus pushes it against the rock. The two wedges are connected with each other by an elastic element so that they can be inserted together into the borehole. The disadvantage is that, in addition to the functional tension forces, bending forces also act on the tension element by means of which the possible tension load is reduced.
According to another suggestion in Swiss Pat. No. 564,654, the anchoring element is designed as a shapeable body which rests in the borehole in its reshaped state. The anchoring element is designed as a hollow body and the tension element is fastened in a closing plate underneath the hollow body. A viscous substance is pushed into the hollow body through the tension element which is designed as a tube so that its shape conforms exactly to the borehole. With such a tie rod, the friction of the hollow body is limited by both the tension element and the borehole. The hollow body consists of a shapeable sleeve, and an additional force limitation is thus given by the rigidity of this sleeve and the tension element cannot be utilized up to its own load capability.
In the rock anchoring unit according to German Pat. No. 2,903,694 a spreading sleeve held in the borehole in a clawlike manner is placed on the tension element and can be tightened through a spreading bolt by turning the tension nut. For this purpose, the ends of the tension elements are conically expanded in order to receive a spreading wedge. When the spreading bolt is designed with a star-shaped cross-section and the points engage in the gap in the tension element effecting the conic expansion, the material, particularly the glass fibers in a synthetic resin tube, cannot turn aside and the strength is increased. However, it has been determined that such a glass fiber synthetic resin tube (GFK) is not held to a sufficient extent by the radial pressure between the spreading bolt and the spreading sleeve, and can therefore slide out. Even when pouring additional epoxy resin, no essential improvement is obtained.
A further problem is created by the threaded portion of the tension rod exerting a tension force by means of a tightening nut.